1. Field of the Invention
The present invention relates to a staging fuel nozzle of a gas turbine engine, and more particularly to a staging fuel nozzle that further increases combustion efficiency in a low or medium load mode of the engine and contributes to further reduction in NOx in a medium or high load mode of the engine.
2. Description of Related Art
A rich-lean combustion system based on diffusion combustion has been used in combustors of conventional jet engines for aircrafts. With such a combustion system, the correlation between the equivalent ratio φ and NOx generation amount has an almost symmetrical convex curved distribution with a center close to φ=1. Therefore, by conducting combustion in a rich state (fuel-rich state) with φ>1 in the upstream portion inside the combustor and then introducing air, it is possible to include combustion in a lean state (fuel-lean state) with φ<1 and avoid the combustion in the vicinity of φ=1, whereby suppressing the generation of NOx. A variety of technological improvements have been made to reduce NOx further, but the NOx reduction effect shows sighs of saturation. Further, a transition to a higher pressure ratio aimed at reduction in fuel consumption will inevitably increase abruptly the emission of NOx and smoke in a rich-lean combustion system.
In order to resolve this problem, a staging fuel nozzle that uses a diffusion combustion system in a pilot fuel injection portion and a premixing combustion system in a main fuel injection portion has been researched and developed extensively (see, for example, Japanese Patent Application Laid-open No. 2002-139221). With such a combustion system, fuel is premixed with a sufficient amount of air and the mixture is subjected to lean combustion in the main burner in order to prevent the appearance of high-temperature flame and reduce NOx generated in a large amount during high-temperature combustion. Accordingly, the pre-mixture for main combustion has to be supplied to combustion in a state in which fuel is sufficiently atomized and sufficiently homogeneously mixed with air.
In the main air flow channel of the staging fuel nozzle, a swirler provided around the pilot fuel injection portion and causing the air flow to swirl in the inlet of each air channel flow has a double annular configuration, and the swirlers are partitioned by an oil film forming body of a cylindrical structure called a film lip. A fuel injection port, which injects the fuel, is provided in the inner wall surface of the main air flow channel, the fuel moves downstream, while colliding with the film lip and forming a liquid film, and is stretched into a thin film by the air flow at the film lip tip and separated, whereby enhancing the atomization of fuel and uniform mixing of air and fuel (see, for example, Japanese Patent Application Laid-open No. 2004-226051). However, in the medium load mode of the engine in which the main injection portion starts operating, the injection speed of fuel is low. Therefore, a major portion of the fuel cannot reach the film lip and flows along the inner wall surface of the air channel. As a result, in the medium load mode of the engine, the fuel is mixed with the air and supplied to combustion, while still being insufficiently atomized. As a consequence, the combustion becomes unstable, a diffusion combustion mode is realized, and a large amount of NOx is generated. A fuel injection valve is known (see, for example, Japanese Patent Application Laid-open No. 2005-180730) in which in order to resolve this problem, the atomization lip (film lip) of the main fuel flow channel has a double annular configuration (the inlet of the main air flow channel has a triple annular configuration), a fuel injection port is provided in the outer circumferential surface of the inner atomization lip, the atomization of the fuel and homogeneous mixing of air and fuel are enhanced by the outer atomization lip in a high load mode of the engine, and the atomization of the fuel and homogeneous mixing of air and fuel are enhanced by the inner atomization lip in a medium load mode of the engine.